This invention relates to a scanning optical apparatus in which a light beam emitted from a light source is reflected by the mirror surface of a rotatable polygonal mirror and thereby deflected and scanned.
As a scanning optical apparatus, there is known one as shown, for example, in FIG. 1 of the accompanying drawings wherein a laser beam LB emitted from a laser source 50 is modulated on the basis of predetermined image information and this modulated beam LB is collimated by a collimator lens 52 and thereafter is reflected by a rotatable polygonal mirror 54 rotated at a uniform speed, whereby the beam LB is scanned in uniform speed circular motion and further the scanning in uniform speed circular motion is converted into scanning in uniform speed rectilinear motion by an f.theta. lens 56 and the beam is imaged on a photosensitive drum 58 to thereby record an image.
In a scanning optical apparatus of this kind, it would occur to mind to rotate the rotatable polygonal mirror 54 at a high speed or increase the number of the mirror surfaces of the polygonal mirror 54 in order to scan the beam at a high speed, but scanning at a higher speed has been difficult because in the mechanical structure of a rotating mechanism, there is a limit in the increase in rotational speed or because the increase in the number of the mirror surfaces makes the polygonal mirror 54 bulky.
Therefore, there would occur to mind a scanning optical apparatus as shown in FIG. 2 of the accompanying drawings where a beam LB emitted from a laser source 62 is caused to pass through a collimator lens 64 and impinge on a rotatable polygonal mirror 66 at an angle .theta. with respect to a normal N on the mirror surface 66a of the polygonal mirror 66 and the beam LB reflected by the mirror surface 66a is turned back to the polygonal mirror 66 by a fixed reflecting mirror 68 disposed in opposed relationship with the polygonal mirror 66 and is reflected twice by the mirror surface 66a of the polygonal mirror 66 and enters an f.theta. lens (not shown).
In this apparatus, the beam is scanned by being reflected twice by the rotatable polygonal mirror 66 and therefore the scanning angle of the laser beam LB is enlarged to twice (when the angle of rotation of the rotatable polygonal mirror is .theta., in the case of one reflection, the incident light and the emergent light form an angle of 2.theta. therebetween, but if the beam is reflected twice by the polygonal mirror, the first incident light and the last emergent light form an angle of 4.theta. therebetween, and this is simply verified by geometrical optics and consequently, the scanning angle becomes twice as great and thus, the scanning speed of the laser beam LB can be enhanced without the number of the mirror surfaces of the polygonal mirror 66 being increased and without the rotational speed of the polygonal mirror 66 being increased.
Also, the number of the mirror surfaces can be made relatively small and therefore, each mirror surface 66a can be made large and the diameter of the beam can be made large and the diameter of the spot on the photosensitive drum can be made small.
In the above-described apparatus, however, the laser beam LB is caused to impinge on the polygonal mirror 66 at an angle .theta. with respect to the normal N to the polygonal mirror 66 and therefore, there arises the problem that as the scanning angle of the laser beam becomes greater, the scanning line on a medium to be scanned becomes curved or the diameter of the laser spot varies.
So, an object of the present invention is to provide a scanning optical apparatus in which, without the apparatus being made bulky, a light beam can be scanned at a high speed and which can obtain a formed image free of distortion.
According to the present invention which achieves the above object, in a scanning optical apparatus a light beam emitted from a light source is reflected by a plurality of mirror surfaces of a rotatable polygonal mirror and thereby deflected and scanned. Each mirror surface of the rotatable polygonal mirror comprises a pair of reflecting surfaces inclined toward the center axis of rotation of the rotatable polygonal mirror and orthogonal to each other. In opposed relationship with one of said pair of reflecting surfaces, there is fixed a reflecting mirror having at least one reflecting surface for reflecting the light beam from said one reflecting surface back to the reflecting surface of the polygonal mirror.
Specifically, the fixed reflecting mirror may be a plane mirror having a reflecting surface or a roof type reflecting mirror, and the light beam is directed from the light source to the rotatable polygonal mirror through the intermediary of a reflecting mirror or a polarizing beam splitter and a .lambda./4 plate.